For many applications, and particularly in consumer electronic devices, the relatively large and heavy cathode ray tube (CRT) has been replaced by flat panel display types such as liquid crystal display (LCD), plasma, electroluminescent and organic light emitting diode (OLED). Flat panel displays are typically used as video screens for a variety of consumer electronics devices, such as televisions, desktop computers and mobile or portable devices such as smart phones, digital audio and video players, video game handsets, and tablet computers. In addition to having a relatively thin profile, flat panel devices typically use less power than the CRT and are also much lighter. The flat panel display contains thousands or millions of display elements or pixels that are formed on a transparent substrate (e.g., glass), where each display element receives a data value or a signal that represents a digital picture element that is to be displayed at that location. With active matrix devices, the signal is applied using a transistor (that may be deemed part of the pixel or display element) that has been formed on the transparent substrate. These are sometimes referred to as thin film transistors (TFTs). The transistor may be driven to act as a switch element, with one carrier electrode that receives the data value, another carrier electrode that applies the data value to the pixel, and a control electrode that receives a gate or scanning signal. The gate signal may serve to modulate, typically turn on and turn off, the transistor so as to write and then store the data value into the pixel.
The array of pixels is overlaid with a grid of conductive data lines and gate lines. The data lines serve to deliver the data values to the carrier electrodes of the transistors, while the gate lines serve to apply the gate signals to the control electrodes of the transistors. In other words, each of the data lines is coupled to a respective group of pixels, typically referred to as a column, while each of the gate lines is coupled to a respective row of pixels. Each data line is coupled to a data line driver circuit that receives control and data values in digital form, from decode and timing logic that may be part of a display driver or controller-integrated circuit. The latter has translated incoming video information from another processor, including digitized pixel values, for example red, green and blue digital pixel values, into data signals having the appropriate timing and voltage and current levels. The pixel array is driven in a row-by-row or scanning line manner, where gate lines are sequentially pulsed, while during assertion of the pulse the desired pixel values are written into each selected row of pixels.
For a given pixel, the amount of light that can be viewed by the user at that point depends on the data value that has been written. Typically, a data line voltage is written as an analog pixel voltage that may be stored by a small capacitor in the pixel. In a TFT active matrix LCD pixel, the data line voltage is applied to a liquid crystal capacitor, to develop a voltage difference between a pixel electrode and a common electrode of the capacitor. This pixel voltage aligns the liquid crystal molecules that are between those electrodes in a predefined way so that light transmission is modulated at that point appropriately. This is also referred to as setting an analog voltage that represents the data value (typically, a digital gray level between white and black), into the pixel.
However, when the gate signal turns off the transistor, the fall time (pulse decay time) of the gate signal may couple onto the capacitor in the pixel. This coupling may cause a slight bump in the voltage across the transistor or cause a charge ejection from the transistor. Accordingly, as the gate signal swings downwards to turn off the transistor, coupling due to parasitic elements (e.g., a parasitic capacitor across the transistor) may occur that affect the voltage being stored in capacitor. Thus, the data line voltage being stored in the capacitor in the pixel may not be accurate because of this coupling.
One way to overcome this effect of the coupling is to reduce the fall time to an optimal fall time that is determined and set in the factory. But, one issue with this solution is that the voltage threshold of the transistor and the gate driver may shift which will cause the fall time of the gate driver to shift as well.